A power conversion apparatus is in growing demand attributable to current measures for controlling carbon dioxide (CO2) emission, development in smart grid, and the like. The growth will continue also in future. The power conversion apparatus is conventionally equipped with multiple power semiconductor devices such as a bipolar junction transistor (BJT), an insulated gate bipolar transistor (IGBT), a metal-oxide-semiconductor field effect transistor (MOSFET), and a free-wheeling diode (FWD).
The power conversion apparatus is accompanied by no small energy loss. Then, decreasing the loss is a problem to be solved not only in the past but also at present. In order to decrease the loss in the power conversion apparatus, power semiconductor devices equipped therein have historically been replaced from BJTs, which belong to a current driven type with higher loss, to IGBTs or MOSFETs, which belong to a voltage driven type with lower loss. Further, among these devices equipped in the power conversion apparatus, IGBTs attract attention, in particular, because of high speed switching features and voltage driven characteristics that MOSFETs possess along with low on-state voltage characteristics that bipolar transistors possess.
Moreover, with regard to a method for fabricating devices such as an IGBT and a MOSFET equipped in the power conversion apparatus, improvements have been achieved one after the other in size reduction processes, wafer thinning processes, and the like, causing the loss of the device itself to decrease furthermore. Along with this, miniaturizing the device and lowering the costs are also in progress simultaneously. As a result, an application range for these devices is enlarged from industrial fields such as a general-purpose inverter, an AC servo, an uninterruptible power supply (UPS), and a switching power supply to consumer appliance fields such as a microwave oven, a rice cooker, and a stroboscope.
On the other hand, in the power conversion circuits which perform AC (alternating current)/AC conversion, AC/DC (direct current) conversion, and DC/AC conversion, a matrix converter attracts attention as a direct conversion circuit which can eliminate a direct current smoothing circuit configured by an electrolytic capacitor, a direct current reactor, and the like. The matrix converter is employed under AC voltage. A plurality of switching devices used as components of the matrix converter require a bi-directional switching device, in which current is controllable for both forward and reverse directions, having bi-directional electrical characteristics. An IGBT having breakdown voltage characteristics bi-directionally for both forward and reverse direction (hereinafter, described as “reverse blocking IGBT”) is known as such a bi-directional switching device.
Making up the device configuration in which the reverse blocking IGBTs are connected in anti-parallel eliminates reverse blocking diodes required if conventional IGBTs are employed to configure a bi-directional switching element, enabling the loss of the bi-directional switching device to decrease. Moreover, lowered loss of the bi-directional switching device can achieve miniaturization, weight reduction, efficiency increase, high-speed response, cost reduction, and the like for the matrix converter. In this context, market demand for reverse blocking IGBTs has increased in recent years. The reverse blocking IGBTs have highly reliable characteristics even for the reverse breakdown voltage in addition to normal forward breakdown voltage. It is required to provide the reverse blocking IGBTs having such characteristics with lower costs.
A reverse blocking IGBT 100 which has a sectional structure at the end portion of a semiconductor substrate (chip) shown in FIG. 5 is known as a conventional reverse blocking IGBT (for example, see the following Patent Literature 1). FIG. 5 is a sectional view illustrating an essential structure of the conventional reverse blocking IGBT. An earlier reverse blocking IGBT (for example, see FIG. 14 in the following Patent Literature 2) than the reverse blocking IGBT 100 requires a deep diffusion layer (p-type isolation layer) which stretches from the front surface to the back surface across a semiconductor substrate. Forming the deep diffusion layer (p-type isolation layer), however, is accompanied by many undesirable problems (characteristic failures and cost increase). Then, the practical use is known to be low.
So, as far the reverse blocking IGBT 100 shown in FIG. 5 is concerned, instead of the p-type isolation layer having the conventional deep diffusion layer, configuring a shallower p-type isolation layer 4 formed from the front surface of the substrate to the predetermined depth thereof reduces the problems occurring in the reverse blocking IGBT with the p-type isolation layer having the conventional deep diffusion layer, raising the practical use. In the reverse blocking IGBT 100 with such a p-type isolation layer 4, a V-shaped groove 8 is formed from the back surface side opposite to the p-type isolation layer 4 having a depth that the base portion thereof contacts the bottom portion of the p-type isolation layer 4. A p-type collector 9 is formed at the back surface flat portion surrounded by the V-shaped groove 8. A p-type thin-layer 11 is formed along the inside face (side wall portion 10) of the V-shaped groove 8. The p-type thin-layer 11 is in contact with the p-type isolation layer 4 and the p-type collector layer 9.
Since the p-type thin-layer 11 contacts the p-type isolation layer 4 and the p-type collector layer 9 with the same conductivity-type, the p-type isolation layer 4 has a similar function to the p-type isolation layer having the deep diffusion layer described above. Configuring the reverse blocking IGBT 100 with such a p-type isolation layer 4 not only eliminates the formation of the p-type isolation layer including a deep diffusion layer which needs conventional diffusion for long hours at a high temperature, but also allows demerits to be avoided for a decrease in breakdown voltage owing to generation of donors in a n− drift layer 1 accompanied by the diffusion for long hours at a high temperature, an increase in leak current owing to occurrence of crystal defects, deterioration in throughput of facilities, and the like. Numeral 5 indicates a guard ring; numeral 6 indicates a field insulation film; numeral 7 indicates a field plate; and numeral 12 indicates a collector electrode.
In the meantime, with regard to a technique for forming a collector electrode of an IGBT, there is provided that reverse breakdown voltage failures caused by what aluminum (Al) spiking occurs can be reduced. The content is as follows. A collector electrode including an Al—Si (aluminum-silicon) film as a first layer is formed on a surface of a collector layer. The aluminum silicon film has a thickness of 0.3 to 1.0 μm and a silicon concentration of 0.5 to 2 wt. %, preferably 1 wt. % (for example, see the following Patent literature 2).
The aluminum spiking phenomenon is explained so that rising in temperature during soldering between a collector electrode and a joined member in assembly process mounting a chip causes mutual diffusion between silicon atoms in a silicon substrate and aluminum atoms in an Al—Si film contacting the silicon substrate directly among the metal films constituting the collector electrode on the back surface of the chip, and then aluminum atoms deposit in the micro recess where silicon atoms leave from the silicon substrate. The aspect is called “aluminum spiking”. The phenomenon, although related to the temperature in soldering, occurs easily when the silicon concentration is low or nothing in the Al—Si film. And, if a pn junction between an n drift layer and a p-type collector layer is shallow in depth from the back surface of the substrate, the aluminum spiking occurring on the back surface of the substrate easily reaches the pn junction. There is a problem that the reverse breakdown voltage characteristics deteriorate.
Further, regarding a technique for forming a collector electrode of an IGBT, there is provided that forming a metal film including nickel (Ni) film of 0.6 to 0.8 μm in thickness for a collector electrode allows wafer warp to be reduced and permits failures owing to cracks and scratches introduced in a wafer to be reduced when transferring wafers and the like (for example, see the following Patent Literature 3).